The present invention relates to an electronic control apparatus for use with an internal combustion engine using a digital computer, and more particularly to an engine speed sensing device wherein crank angle pulses are counted by a counter for a set time interval and the crank angle pulse count is read into a central processing unit of the computer to control, for example, the respective amounts of intake air and fuel supplied, spark timing and so forth.
In a conventional electronic control apparatus, a time interval set by the central processing unit (CPU) is counted by clock pulses. Angle pulses produced by a crank sensor each time the crankshaft rotates through a predetermined angle are counted by a counter during the time interval, and are latched at the end of the time interval. Then the latched count of angle pulses is read by the CPU after a subsequent time interval also set by the CPU. In other words, the timing at which the latched count of angle pulses are read into the CPU does not coincide with the end of the corresponding set time interval. Thus, the presence of this difference between the end of each set time interval and the time the latched count of angle pulses is read ensures some imprecision in the crank angle pulse count used to determine engine speed. Thus, if the engine speed greatly changes between the end of the immediately preceding time interval and the time of reading of the latched count of angle pulses, the latest data read into the CPU will not accurately reflect actual conditions, so that precise, high-speed control of the amounts of intake air and fuel supplied and so forth cannot be ensured.